1. Field of the Invention
The invention relates generally to a process for polyolefin manufacturing in gas-phase fluidized bed polymerization reactors.
2. Background Art
Gas phase fluidized bed reactors for the production of olefin polymers are well known in the art. Gas phase processes successfully allow for production of a vast array of polymers, while reducing energy requirements and capital investments required to run the gas phase processes as compared to other polymerization processes.
Gas phase polymerization processes typically run a continuous cycle of a gaseous stream through the reactor. Generally, the stream contains one or more monomers. The stream is continuously passed through the fluidized bed under reactive conditions in the presence of a catalyst. The stream is withdrawn from the fluidized bed and recycled back into the reactor. Simultaneously, polymer products are withdrawn from the reactor and additional monomer is added to the stream to replace the polymerized monomer. In gas phase fluidized bed polymerizations, the polymer products are discharged from the reactor in a granular form. As compared with the polymer products from other types of reactors (e.g., slurry reactor, solution reactor), dry granular particles advantageously allow for easy flow and transportation, without need for removal of solvents and/or catalysts.
By continuously flowing the stream of monomers through the reactor under reactive conditions, thereby exposing the monomers to catalysts present in the reactor, polymerization of the monomers occurs. The polymer products result from the formation of “micro-particle clusters” on the activation sites of the catalyst particles. As the micro-particle clusters develop, spaces are often present between the clusters. These spaces lead to voids of space in the polymer granular particles as the micro-particle clusters grow and develop into granular polymer “macro-particles.” For example, in polyethylene particles made in a gas phase reactor, there may often exist a void of 10 to 25 percent by volume.
The size of voids present in a granular polymer particle may partially depend upon the activity of the catalysts in the fluidized bed reactor. A sudden halt of catalytic activity may contribute to the existence of voids. Such a halt may result for example from a rise in temperature such that the temperature exceeds the catalyst's threshold temperature for activity. Such heat may be generated from the polymerization process itself. Inadequate removal of this heat generated from the polymerization process may further result in temperature gradients within the growing polymer particle. See S. Floyd, et al., “Polymerization of Olefins through Heterogeneous Catalysis III. Polymer Particle Modelling with an Analysis of Intraparticle Heat and Mass Transfer Effects,” J. App. Polymer Sci, vol. 32, 2935-60 (1986). W. H. Ray, et al., “Polymerizaton of Olefins through Heterogeneous Catalysis X: Modeling of Particle Growth and Morphology,” J. App. Polymer Sci., vol. 44, 1389-1414 (1992) also teaches that greater heat and mass transfer resistance may lead to higher internal voids within granular polymer particles. Significant polymer particle overheating has also been hypothesized as a cause for particle sticking and agglomeration problems in gas phase polymerizations.
The existence of the voids in the polymer often necessitates that the polymer granules undergo a high-energy consumption pelleting procedure, whereby the granular particles are melted to produce pellets having a density similar to that of the polymer density and a desired size. When there is no void in polymer pellets, the density of the pellets will be identical to the polymer density. Such pellets are often desired by customers as they allow for efficiency in transportation and handling. The pelleting procedure, however, contributes significantly to manufacturing and operating costs.
When the granular particle density of the polymer granules discharged from the reactor is relatively similar to the polymer density, the pelleting procedure can be eliminated. Granular particles that are discharged with the proper particle size and/or particle size distribution can be delivered directly to the customers after purging out residual hydrocarbons.
Minimization of void space and thus maximization of bulk density or granular particle density may allow for an increase in reactor inventory, in which case a given reactor would be equivalent to a larger reactor having a higher production capacity, with fewer costs and time associated with a pelleting procedure that can either be improved or eliminated. Accordingly, there exists a need for a polymerization process by which polymer particles having a less void and a greater granular particle density may be achieved.